This multidisciplinary proposal seeks to develop and validate a novel class of cancer imaging biomarkers for rapid clinical deployment. Leveraging advances in basic and clinical cancer biology, innovative medicinal chemistry, and high-throughput, small molecule screening, our laboratory has identified tumor-selective translocator protein (TSPO) ligands that are promising candidates for further development as positron emission tomography (PET) imaging agents. TSPO is an 18 kDa high-affinity cholesterol- and drug-binding protein that participates in cholesterol metabolism, steroid biosynthesis, proliferation, and apoptosis. Elevated levels of TSPO are well-documented in oncology, where levels correlate with tumor proliferation, invasion, and metastasis. Furthermore, our laboratory has mechanistically linked TSPO ligand binding to mitogen-activated protein kinase (MAPK) signaling in glioma. Capitalizing upon this observation, we have shown that TSPO ligand PET can be used to evaluate MAPK pathway inhibition in this setting. Collectively, these data demonstrate the potential significance of TSPO ligand PET as a predictive imaging biomarker in oncology. The overall goal of this study is to develop clinically translatable, tumor-selective TSPO imaging ligands that facilitate improved detection, molecular characterization, and prognosis of human gliomas. Our Specific Aims are [1] to optimize novel, tumor-selective TSPO ligand leads for in vivo PET imaging, [2] to develop methods for labeling promising TSPO ligands with the positron-emitting isotope fluorine-18, and [3] to evaluate promising TSPO imaging ligands in vivo.